Spectacle lenses are being manufactured from an increasingly wide number of materials having varied spectral characteristics. It has become important not only to know the power of such lenses but to be aware of the lenses spectral absorption characteristics. The reasons for knowing the spectral characteristics of such lenses can be easily summarized.
First, and most importantly, the ophthalmic community has become increasingly aware of the importance of the ultraviolet absorption characteristics of spectacle lenses and sunglasses. Simply stated, ultraviolet radiation in unacceptably large exposure can be harmful to the eye. Unfortunately, with modern glasses utilized in spectacle lenses, such absorption characteristics vary widely. Glasses are known which diminish light in the visible spectrum while having reduced or little effect on light in the ultraviolet. This characteristic of lenses causes the pupil of the eye to dilate to the maximum aperture to accommodate light in the visible spectrum--letting in what can be harmful amounts of ultraviolet radiation.
Secondly, spectral readings on the absorption characteristics of spectacle lenses can be a convenient way to measure the color of the lenses for either order or comparison. Unfortunately, eye glasses now come in so many different shades or colors that meaningful comparison or specification of the material in terms of their measurable spectral characteristics is not possible. Consequently, there is no way to "compare" quantitatively the color of eye glasses. More over, there is no acceptable way to specify eye glasses by their color.
Thirdly, when the material of such eye glasses is measured for its spectral characteristics, the eye glass is already configured into a lens. This being the case, in all known spectrometers, the power of the lens effects the spectral measurement of the glass of the eye glasses. Since prescriptions vary widely, the impact of the prescription upon the measurement of the spectral characteristics of the glass is not predictable--especially when both the power of the lens and the absorption characteristics of the lens both constitute unknowns at the beginning of measurement.